The invention relates to an apparatus for transforming a semiconducting thin layer, and more particularly to an apparatus for transforming a semiconducting thin layer, utilizing a laser crystallization method (a method wherein an amorphous semiconductor material deposited on a substrate is crystallized with laser irradiation).
Field-effect thin film transistors (TFTs) provided on a glass substrate or the like have hitherto been extensively used, for example, as driving elements for displays, sensors, and printing devices. Representative technology for forming such TFT include a hydrogenated-amorphous silicon TFT technology and a polycrystalline silicon TFT technology.
In the former method, the highest temperature of the preparation process is about 300xc2x0 C., and a carrier mobility of about 1 cm2/Vsec is realized. This method is used in the production of switching transistors for each pixel in active matrix (AM-) liquid crystal displays (LCDs). AM-LCD has a driving TFT for each pixel and drives TFT of each pixel is driven by peripheral integrated circuit around the display area(IC; LSI formed on a monocrystalline silicon substrate). Since a switching TFT is provided on each pixel, as compared with passive matrix LCD wherein a liquid crystal driving electric signal is sent from a peripheral driver circuit, AM-LCD features a reduction in cross talk and the like and the realization of good image quality.
On the other hand, the latter method uses, for example, a quartz substrate, and the use of a high-temperature process similar to LSI using a temperature of about 1,000xc2x0 C. can provide a performance of 30 to 100 cm2/Vsec in terms of carrier mobility. The realization of this high carrier mobility, when this method is applied, for example, to liquid crystal displays, enables pixel TFT for driving each pixel and a peripheral drive circuit section to be simultaneously formed on a single glass substrate. This can lead to advantages associated with a reduction in production process cost and a reduction in size of LCD. That is, connecting TFT to peripheral drive circuits through tab connection or wire bonding according to a prior art method is difficult to cope with a reduction in pitch of connection between the AM-LCD substrate and the peripheral driver integrated circuit due to the reduction in size of LCD and the increase in resolution. The above polycrystalline silicon TFT method has an advantage of the reduction in size, but on the other hand, when the high-temperature process is used, the inexpensive glass having a low softening point usable in the former process cannot be disadvantageously used.
For this reason, in order to solve the above problems, it is necessary to lower the temperature in the polycrystalline silicon TFT process. This has led to enthusiastic research and development of a method for polycrystalline silicon layer formation at a low temperature that has applied a laser crystallization method. For example, Japanese Patent Publication No. 118443/1995 discloses a method wherein a short-wavelength pulse laser beam is applied to crystallize a thin layer of amorphous silicon provided on an amorphous substrate and the crystallized thin layer is applied to thin film transistors. According to this method, amorphous silicon can be crystallized without raising the temperature of the whole substrate. Therefore, a semiconductor element or a semiconductor integrated circuit can be advantageously prepared on a large area substrate, such as a liquid crystal display, and an inexpensive substrate such as glass. As described in the above publication, however, an irradiation intensity of about 50 to 500 mJ/cm2 is necessary for the crystallization of a thin layer of amorphous silicon by a short-wavelength laser beam.
On the other hand, the maximum light emission output in a currently generally available pulse laser device is about 1 J/pulse, and, upon simple conversion, the area over which the laser beam can be applied per single laser beam irradiation is as small as about 2 to 20 cm2. Therefore, for example, a laser beam should be applied to at least 87 to 870 sites for crystallizing the whole substrate having a size of 47xc3x9737 cm2 by the laser beam. When the size of the substrate is increased, for example, to 1 m square, the number of laser irradiation sites should be increased accordingly. Further, an attempt has also been made wherein a linear beam (length 100 to 300 mm, width about 1 to 0.1 mm) is used as the beam applied to the substrate and the beam is scanned in the direction of width to change the beam scanning direction from two axes (x axis and y axis) to one axis (x axis).
In general, the laser crystallization is realized by a pulsed laser irradiation device having a construction shown in FIG. 14.
FIG. 14 is an explanatory view of a conventional ELA apparatus. As shown in the drawing, a laser beam fed from a pulsed laser beam source 1401 reaches a thin layer 1407 of silicon on a glass substrate 1408 as an irradiation object through optical paths 1406 specified by a group of optical devices, such as mirrors 1402, 1403, 1405 and a beam homogenizer 1404 provided for homogenizing spatial intensity. In general, since one irradiation area range is smaller than the glass substrate, a laser beam is applied to any desired site on the substrate by moving the glass substrate 1408 on an x-y stage 1409. A construction may also be adopted wherein, instead of the use of the x-y stage 1409, the group of optical devices is moved, or alternatively the group of optical devices is used in combination with the stage.
A linear beam irradiation form having a length equal to the size of one side of the substrate is adopted, and this beam can also be applied while moving a Y stage with the substrate disposed thereon. In this case, the movement of the stage and the feed of the pulsed beam are carried out according to the following procedure.
1) Simultaneously with the movement of the stage at a given rate, a pulsed laser beam is oscillated and fed at a given cycle.
2) (Movement of the stage by one step followed by stop)+(the feed of the pulsed laser beam by one pulse) are repeated.
FIG. 15 is a diagram illustrating a conventional irradiation method in the case where the beam irradiation form is rectangular. In general, since one irradiation area range 1502 is smaller than the glass substrate, a laser beam is applied to any desired site on the substrate by moving the glass substrate 1501 on an x-y stage. A construction may also be adopted wherein, instead of the use of the x-y stage, the movement of a group of optical devices (for example, direction X) is utilized in combination with the movement of the stage (direction Y). Crystallized regions 1503 are successively formed by adopting this method. In this case, the movement of the stage and the feed of the pulsed beam are carried out according to the following procedure. Numeral 1504 designates a laser introduction window provided in the chamber.
3) Simultaneously with the movement of the stage at a given rate, the pulsed laser beam is oscillated and fed at a given cycle.
4) (Movement of the stage by one step followed by stop)+(the feed of the pulsed laser beam by not less than one pulse) are repeated.
In the laser crystallization using a linear beam or a rectangular beam, means for moving a substrate stage is utilized.
In some cases, the laser beam irradiation is carried out within a vacuum chamber in vacuum or in a high-purity gas atmosphere. If necessary, as shown in FIG. 14, a system may be adopted wherein a glass substrate-containing cassette 1410 provided with a thin layer of silicon and a substrate-carrying mechanism 1411 are provided and the taking-out and the housing of the substrate are mechanically carried out between the cassette and the stage. In the adoption of the above method, the glass substrate has been merely disposed on the xy stage, and any special means for fixing and holding the substrate on the stage has not been adopted.
In order to enhance the utilization efficiency of the laser beam, however, it is necessary to apply the laser beam to only a desired region while avoiding the application of the laser beam to a portion unnecessary to be irradiated with the laser beam (for example, a cut margin created at the time of cutting of devices), thereby reducing the number of laser beam irradiation sites and enhancing the utilization efficiency of the laser beam. To this end, the device preparation region should conform to the laser irradiation region, and the accidental shift of the substrate on the stage should be prevented. In particular, when the stage has a coordinate system and the irradiation site is controlled based on this coordinate system, the accidental shift should be avoided. Further, deflection of the glass substrate upon the formation of a semiconducting thin layer or heating of the substrate results in defocusing. Therefore, the substrate should be intimately contacted with the stage from the viewpoint of correcting the deflection.
Accordingly, it is an object of the invention to solve the above problems of the prior art and to provide an apparatus for modifying a semiconducting thin layer, which can apply a laser beam to only a desired region through the prevention of accidental shift between the stage and the substrate.
In order to attain the above object, according to the invention, an apparatus for modifying a semiconducting thin layer, comprises:
a hermetically sealed container comprising a substrate mount section for mounting thereon a substrate with an amorphous semiconducting thin layer formed thereon, and a window for introducing a laser beam;
laser beam irradiation means which is provided outside the hermetically sealed container and applies a laser beam through the window, for heat-melting the amorphous semiconducting thin layer;
holding means for fixing and holding the substrate on the substrate mount section; and
pressure control means which regulates the flow rate of gas fed into the hermetically sealed container to control the pressure of the atmosphere within the hermetically sealed container at the time of irradiation with the laser beam at a value above a vapor pressure specified by the temperature of the heat-melted amorphous semiconducting thin layer.
The invention includes the following preferred embodiments.
Specifically, the holding means may be vacuum adsorption means.
The holding means may be electrostatic adsorption means.
The substrate may be a glass substrate, and the amorphous semiconducting thin layer may be a thin layer of amorphous silicon.
The apparatus may further comprise: means for introducing nitrogen or inert gas into the hermetically sealed container; and means for introducing oxygen gas into the hermetically sealed container.
The apparatus may be used in the production of a field-effect thin film transistor.
The field-effect thin film transistor may be used as a driving element for active matrix liquid crystal devices.
By virtue of the above construction, the invention can prevent accidental shift of the substrate, and can closely control the irradiation site. Therefore, the number of laser beam irradiation sites can be reduced, and the utilization efficiency of the laser beam can be enhanced. In particular, in the case where the stage has a coordinate system and the laser beam irradiation site is controlled based on this coordinate system, the effect of preventing accidental shift is large. Further, the substrate can remain fixed onto the stage even under conditions such that the glass substrate is deflected upon the formation of a semiconducting thin layer and heating of the substrate. Therefore, the correction of the deflection is possible. This can realize the prevention of defocusing of the laser beam.